Method of anodizing aluminum



United States Patent Pennsylvania N0 Drawing. Filed May 2, 1963, Ser. No. 277,471 6 Claims. (Cl. 204-58) This invention relates to anodizing aluminum, and particularly to a novel method for producing anodic oxide coatings on aluminum. In this method an aluminum article is made anode in an electrolytic cell employing an aqueous acid electrolyte as described below. As generally used herein, the word aluminum includes high purity aluminum, aluminum in various commercial grades, and aluminum base alloys.

Decorative and protective anodic oxide coatings on aluminum have long been made by anodizing in electrolytes consisting of aqueous solutions of sulfuric acid. Such electrolytes are usually employed at a temperature near room temperature, and must be cooled to maintain such temperature. The anodic oxide coatings, as so produced, are usually relatively clear or colorless, although on sirne alloys they may be tinted or colored by various alloying constituents.

In recent years, so-called hard anodic oxide coatings on aluminum, coatings which are more abrasion resistant, have been produced by anodizing in aqueous solutions of sulfuric acid at extremely low temperatures, e.g. 25 to 32 F., requiring much more cooling with consequent increased equipment and operating expense. Such coatings, as so produced, in addition to being abrasion resistant, are colored in metallic gray, brownish gray and dark brown colors, which are attractive for architectural and other uses of aluminum. Consequently, more practical methods of producing similarly-colored anodic oxide coatings have been in demand.

It is a general object of this invention ot provide, for use in an electrolytic cell for anodizing aluminum, a novel electrolyte and anodizing process or method for producing anodic oxide coatings on aluminum, particularly colored coatings. Another object is to provide such method for use under a wide range of practical operating conditions. A particular object is to provide such a method for anodizing aluminum in which moderate cooling of the electrolyte is permissible without adverse effect. Another object is to produce abrasion resistant, colored anodic oxide coatings on aluminum at moderate cost. A specific object of the invention is to produce uniform, abrasion-resistant, lightfast, colored anodic oxide coatings on aluminum economically.

In accordance with the invention, it has been found that an electrolyte consisting essentially of sulfoanthranilic acid, together with sulfate provided by at least one compound selected from the group consisting of sulfuric acid and water soluble sulfates and bisulfates, and water, may be advantageously used in an electrolytic cell for anodizing aluminum. As generally used herein, the expression sulfoanthranilic acid includes 4-sulfoanthranilic acid and S-sulfoanthranilic acid, which are water-soluble sulfonic acid-substituted anthranilic acids. Alkali metal sulfoanthranilates may be readily converted to the acid form, for use in accordance with the present invention, by replacing the cations with hydrogen. A conventional ion-exchange or other suitable procedure may be used for this purpose.

The sulfoanthranilic acid content of solutions employed as electrolytes in accordance with the invention may be a rather small amount, e.g. 10 grams per liter. However, somewhat higher sulfoanthranilic acid contents, e.g. at least 50 grams per liter or more, impart greater electrolyte conductivity and are easier to control.

3,243,362 Patented Mar. 29, 1966 The amount used may equal or even exceed the solution limit as an undissolved excess will not prevent anodizing.

The sulfate content of the solutions employed as electrolytes may be small, eg 1 gram per liter (total sulfate calculated as H 80 Somewhat higher sulfate contents, preferably 2 grams per liter or more, will ordinarily be employed, since the voltage requirements for the same anodizing current density (with consequent electrical equipment and electrolyte cooling cost factors to be considered) are reduced in such cases. However, an increase in the sulfate content tends to reduce the depth of color obtained in the anodic coating. For the production of the most satisfactory colored coatings at or near room temperature, therefore, a sulfate content between 2 and 8 grams per liter is desirable. The sulfate content may desirably be provided by sulfuric acid, but an alkali metal or ammonium sulfate or bisulfate such as sodium sulfate, sodium bisulfate or ammonium bisulfate, a heavy metal sulfate or bisulfate such as ferrous sulfate, an organic sulfate or bisulfate such as aniline sulfate, or any other water soluble sulfate or bisulfate such as hydrazine sulfate may be used. Double sulfates may also be used.

During use, alrnuinum dissolves in the electrolyte, which may be removed by ion exchange, or addition of sulfoanthranilic acid may be made to maintain the desired free acid content.

While various proportions of sulfoanthranilic acid and sulfate may be used, it has been found that the production of uniform, colored coatings at or near room temeprature is most readily obtained when the sulfoanthranilic acid content is greater than the sulfates content, by a ratio of 6:1 or more by weight. However, electrolytes with higher amounts of sulfate content than sulfoanthranilic acid are also useful. Generally, the higher the sulfate content the lighter the color obtained in the coatings. However, even with high sulfate containing electrolytes, abrasion resistant coatings are obtained at lower temperatures which need not, nevertheless, be as low as with sulfuric acid electrolytes devoid of sulfoanthranilic acid.

Specific examples of electrolyte make up for use in accordance with the invention are as follows:

Electrolyte A Grams per Component: liter S-sulfoanthranilic acid 63 Sulfuric acid 2 Water Balance A dark bronze coating may be obtained on 6063 alloy in the T5 temper, for example, upon anodizing at a current density of 24 amperes per square foot, in Electrolyte A, at F. for 25 or 30 minutes. Alloy 6063 is an aluminum alloy which nominally contains 0.4 silicon and 0.7 magnesium.

Electrolyte B Grams per Component: liter 4-sulfoanthranilic acid l3 Sulfuric acid 2.5

Water Balance A tan bronze coating may be obtanied on 6063 alloy in the T5 temper, for example, upon anodizing at "a current density of 24 amperes per square foot, in Electrolyte B, at 75 F., for 10 minutes.

As indicated above, electrolytes for use in accordance with the invention may be employed under a wide variety of operating conditions. An aluminum article may be made anode therein at a temperature at or near room temperature, e.g. 75 F., to produce an abrasion resistant, colored anodic oxide coating. However, either extremely low temperatures or rather high temperatures may also be employed i.e. temperatures between about 40 and 110 F. Very satisfactory results in producing colored coatings with Electrolyte A, for example, with a moderate amount of cooling being required, may be obtained with temperatures between about 60 and 90 F.

Current densities may extend over a wide range, for practical purposes as low as 9 amperes per square foot or even less, and as high as 144 amperes per square foot or even more, depending on size, shape and composition of the aluminum article. Preferably, the current density is maintained between 12 and 36 amperes per square foot. The current may be either AC. or D.C., or a current of undulating characteristics, but D.C. is quite satisfactory.

Time of treatment depends upon current density, and thickness and color of coating desired, as coating thickness and colorris generally a function of anodizing current density and time. Abrasion resistant coatings for outdoor service may desirably be made in a thickness of 0.4 to 1 mil (0.0004 to 0.001 inches). However, coatings may be made in any substantial thickness desired, e.g., 0.1 mil or greater.

Illustrative of particularly suitable operating conditions, Electrolyte A above mentioned has been operated at 75 F., at 24 amperes per square foot, with voltages starting at about 38 volts and moderately increasing over a 30 minute period to produce abrasion resistant, colored oxide coatings.

Coatings produced in accordance with the invention ing an aluminum article anode in an electrolyte consisting essentially of at least 10 grams per liter of sulfoanthranilic acid,

at least 1 gram per liter of sulfate (calculated as H 80 provided by at least one compound selected from the group consisting of sulfuric acid and water soluble sulfates and bisul-faies, and

water, for a time suificient to produce an anodic oxide coating of substantial thickness.

2. A method of anodizing aluminum, comprising making an aluminum article anode in an electrolyte consisting essentially of at least grams per liter of sulfoanthranilic acid,

between 2 and 8 grams per liter of sulfate (calculated as H provided by at least one compound selected from the group consisting of sulfuric acid and water soluble sulfates and bisulfates, and I water, for a time sufiicient to produce an anodic oxide coating of substantial thickness.

3. A method in accordance with claim 1 in which the electrolyte is maintained at a temperature between about 40 and F. and

the current density is maintained between about 9 and 144 amperes per square foot.

4. A method in accordance with claim 1 in which the sulfoanthranilic acid is 4-sulfoanthranilic acid.

5. A method in accordance withclaim 1 in which the sulfoanthranilic acid is 5-sulfoanthranilic acid.

6. A method in accordance with claim 1 in which the sulfate is provided by sulfuric acid.

References Cited by the Examiner UNITED STATES PATENTS Re. 25,566 5/1964 Deal et a1. 204-,58 2,918,416 12/1959 Taylor 204-58 3,031,387 4/1962 Deal et al. 204-58 3,098,018 7/1963 Kissin et a1 204-58 X 3,146,178 8/1964 Cochran et a1 204-58 JOHN H. MACK, Primary Examiner.

G. KAPLAN, Assistant Examiner. 

1. A METHOD OF ANODIZING ALUMINUM, COMPRISING MAKING AN ALUMINUM ARTICLE ANODE IN AN ELECTROLYTE CONSISTING ESSENTIALLY OF AT LEAST 10 GRAMS PER LITER OF SULFOANTHRANILIC ACID, AT LEAST 1 GRAM PER LITER OF SLFATE (CALCULATED AS H2SO4) PROVIDED BY AT LEAST ONE COMPOUND SELECTED FROM THE GROUP CONSISTING OF SULFURIC ACID AND WATER SOLUBLE SULFATES AND BISULFATES, AND WATER, FOR A TIME SUFFICIENT TO PRODUCE AN ANODIC OXIDE COATING OF SUBSTANTIAL THICKNESS. 